Complex, carbonaceous materials that are precursors for oil and gas can be found both in nature and in refining operations. One such material is kerogen. In geological terminology, kerogen is defined as organic matter, derived from plant and bacterial remains, dispersed in sedimentary rocks that is insoluble in traditional organic solvents. Kerogens yield hydrocarbons when the sediments undergo destructive distillation. Kerogens, and the sediments that contain them, can comprise what is known as hydrocarbon source rock. Predicting the timing and composition of hydrocarbon evolution from kerogens in source rocks under geological conditions is important for oil and gas exploration and exploitation. Coal, tar sands and bitumen are other examples of complex, carbonaceous materials occurring in nature. Complex, carbonaceous material is also found as a product in petroleum refining operations, know as residuum. Residua are those fractions that are non-distillable under given conditions and remain at the bottom of a distillation tower. Predicting the kinetics and product yields from the thermal decomposition of petroleum residua is important to refining operations e.g., coking processes.
Significant effort has been expended over the years to characterize kerogen from both chemical and physical perspectives. A common way to determine the composition of oil and gas produced from a given kerogen is to experimentally measure the kinetics and compositions of pyrolysis products and use that information to postulate the original chemical structure of the kerogen. During pyrolysis, a sample is rapidly heated, usually under exclusion of air, to a temperature high enough to break some of the chemical bonds. Knowledge of when and where bonds are formed and broken as well as how molecular structures change during a reaction is important in understanding the kinetics. Many laboratory experiments are needed to extract kinetic data and careful analysis of multiple types of analytical data is necessary to follow the compositional path of the generated hydrocarbons as they evolve. Furthermore, the results apply only to the particular kerogen being investigated.
One approach, as applied to coal, has been to use Fourier Transform Infrared spectroscopy (“FTIR”) together with other techniques such as Thermal Gravimetric Analysis (“TGA”), Field Ionization Mass Spectroscopy (“FIMS”) and Carbon13 Nuclear Magnetic Resonance spectroscopy (“NMR”) to develop a chemical structural model of coal. Bond-breaking rules were developed to act on the chemical structural models of the coal to predict volatile organic matter evolution. A thermal-chemistry mechanism, used to describe the bond breaking processes, was simplified to about three to five steps. However, this type of model is not capable of predicting the timing and molecular composition of the hydrocarbon products at the level of detail required for hydrocarbon generation in nature or petroleum residua in refining operations because of oversimplification of either chemical structure representation or thermal-chemical mechanisms.
In the last decade, NMR and X-ray based solid state characterization techniques have progressed significantly toward quantifying average chemical structural properties of carbonaceous solids. Works have been published on the use of solid state 13C NMR to determine parameters relating to carbon skeletal structure such as the average aromatic ring size and the number of attachments per aromatic cluster. X-ray Photoelectron Specrtroscopy (XPS) has been used to determine the functional forms of organic oxygen, sulfur and nitrogen and to determine the percentage of aromatic carbon. X-ray Absorption Near Edge Structure Spectroscopy (XANES) has been developed for sulfur speciation. The information from such direct characterization techniques has been combined to guide construction of chemical structural models for deposits formed in internal combustion engines.
Further, the pyrolysis of a generic asphaltene has been simulated by combining model-compound-deduced thermolysis kinetics and pathways with asphaltene chemical structure information. A stochastic approach, using a Monte Carlo simulation, was applied to the chemical structure information to construct an ensemble of thousands of chemical structures with particular reactive functionalities, where the ensemble average conformed with experimental observables. Such an ensemble has been connected to simple, but well-defined kinetic models. However, these models exclude heteroatoms and have been oversimplified in their development.
In order to extend NMR and X-ray based solid state characterization techniques used to quantify average chemical structural properties of complex carbonaceous materials toward a predictive compositional yields model, a method is needed to expand the average chemical structure to reflect the tremendous molecular diversity, including heteroatoms, within the material. It is this diversity that leads to the complex nature of crude oil and gas. A more realistic hydrocarbon compositional yield model capable of predicting the kinetics and composition of hydrocarbons generated from the thermal decomposition of complex carbonaceous materials, such as kerogen and petroleum residua, is also needed.